Seismic geophysical surveys are used in petroleum, gas mineral and water exploration to map the following: stratigraphy of subterranean formations, lateral continuity of geologic layers, locations of buried paleochannels, positions of faults in sedimentary layers, basement topography, and others. Such maps are deduced through analysis of the nature of reflections and refractions of generated seismic waves from interfaces between layers within the subterranean formation.
A seismic energy source is used to generate seismic waves that travel through the earth and are then reflected by various subterranean formations to the earth's surface. As the seismic waves reach the surface, they are detected by an array of seismic detection devices, known as geophones, which transduce waves that are detected into representative electrical signals. The electrical signals generated by such an array are collected and analyzed to permit deduction of the nature of the subterranean formations at a given site.
Seismic energy sources that have been used in geophysical survey methods for petroleum, gas, copper, coal, diamond and other mining exploration operations include explosives, vibratory sources and impact sources. The nature of output seismic energy depends on the type of seismic energy source that was used to generate it.
Explosive seismic energy sources used in petroleum and gas exploration on land rely on the explosion of material placed within a subterranean formation to generate seismic waves. Typically, a hole is drilled in the ground, the explosive is placed in the hole, and backfill is piled on top of the explosive, prior to initiating the explosion. Compared on a pound for pound basis to other energy sources, explosive sources impart a very high amount of seismic energy into the ground. Explosive seismic energy sources currently being used in geophysical survey methods generally produce waves of very high frequency.
Many explosives used in seismic energy sources generate high gas volumes. This is a useful property in mining for moving rock, but is undesirable in seismic exploration, because it decreases the amount of usable seismic energy that is generated. Explosives that produce high volumes of gas cause much of the energy of the explosion to be lost as expanding gases force backfilled material up the borehole into which the explosive was placed. Thus, less of the energy generated by the explosion is transferred into the subterranean formation than would be theoretically possible if less energy was lost to the expansion of generated gases. Further, as the explosives are considered bombs in certain countries, their use is severely limited.
Vibratory sources are also used as seismic energy sources in geophysical survey methods. Two categories of vibratory sources include those that generate seismic waves originating at the surface and those that generate seismic waves that emanate from downhole. One mechanical-hydraulic vibratory source, the Vibroseis truck, is specially designed to place all of its weight onto a large platform which vibrates. This vibration, in turn, produces seismic waves in the subterranean formation. Vibroseis trucks have been used extensively in geophysical survey methods, not just for the petroleum and gas exploration, but also for studying the evolution and development of specific geological structures (e.g., the Rocky Mountains) and fault lines. Vibratory sources tend to produce highly repeatable seismic energy. The nature of the energy delivered into the ground by vibratory sources, its amount, duration, and time of delivery, can be tightly controlled and therefore the seismic energy generated tends to be very reproducible, which is a benefit. However, vibratory sources are often not suited to certain types of terrain. For example if the ground is very soft, it can be difficult to use Vibroseis trucks as a seismic energy source.
Fundamentally, an impact source is a weight striking the surface of the earth directly or impacting a plate placed on the earth's surface, yielding seismic energy. A weight-drop is an example of the former type of impact source. Impact sources tend to be relatively inexpensive, and simple to operate and maintain. Additionally, they do not bring about many of the disadvantages associated with the former two impact sources. Unfortunately, their principal disadvantage is that they are inefficient at continuously producing seismic energy useful for geophysical survey of deeper layers. Impact sources typically tend to yield a relatively high proportion of low frequency, surface waves and output less seismic energy than other seismic energy sources. Also, traditional impact sources are generally unable to reliably produce shear waves.
Accordingly, there is a need in the art for improved seismic methods and geophysical survey systems that rely on impact sources that convert a higher amount of the potential energy in the impact source into seismic energy and may be used to reliably produce shear waves.